How to see how many emails will be sent
In the upper right-hand corner of the Edit Post screen, you'll see a box named Postmatic Delivery.
There are a few things to notice about the Postmatic Delivery box. First of all, you'll see how many of your subscribers will receive a copy of the post you are working on. This number is not necessarily the total number of subscribers to your blog. It may change slightly because your subscribers are able to define which authors they would like to receive post notifications from, or choose to receive all posts regardless of author.
This number will also change when you publish your post. It serves as an actual indicator of how many emails have been sent. If you have a huge number of subscribers you'll even see it increase over a few minutes as the emails get sent and delivery is confirmed.